In-Situ Formed Capping Layer in MTJ Devices

ABSTRACT

A method of forming an integrated circuit includes forming magnetic tunnel junction (MTJ) layers; etching the MTJ layers to form a MTJ cell; and forming a dielectric capping layer on sidewalls of the MTJ cell, wherein the step of forming the dielectric capping layer is in-situ performed with the step of etching the MTJ layers.

This application is a continuation of U.S. patent application Ser. No.12/032,973, entitled “In-Situ Formed Capping Layer in MTJ Devices,”filed on Feb. 18, 2008, which application is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to semiconductor memory devices,and more particularly to magnetoresistive random access memory (MRAM)devices and methods of manufacturing the same.

BACKGROUND

Semiconductors are used in integrated circuits for electronicapplications, including radios, televisions, cell phones, and personalcomputing devices, as examples. One type of semiconductor device is thesemiconductor storage device, such as dynamic random access memory(DRAM) or a flash memory, both of which use charge to store information.

A more recent development in semiconductor memory devices involves spinelectronics, which combines semiconductor technology and magneticmaterials and devices. The spins of electrons, through their magneticmoments, rather than the charge of the electrons, are used to indicatethe presence of a “1” or “0”. One such spin electronic device is amagnetoresistive random access memory (MRAM) device 100, sometimesreferred to as a magnetic RAM, as shown in FIG. 1, which includesconductive lines (wordlines WL and bitlines BL) positioned in differentdirections, e.g., perpendicular to each other in different metal layers.The conductive lines sandwich magnetic stacks or magnetic tunneljunctions (MTJ) 102, which function as magnetic memory cells. FIG. 1shows a perspective view of a portion of a prior art cross-point MRAMarray 100. The MRAM device 100 includes a semiconductor wafer comprisinga substrate (not shown). The substrate has a first insulating layer(also not shown) deposited thereon, and a plurality of first conductivelines or wordlines WL is formed within the first insulating layer, e.g.,in a first wiring level.

In a cross-point magnetic memory device 100, each memory cell ormagnetic tunnel junction (MTJ) 102 is disposed over and abuts onewordline WL. Each MTJ 102 includes three layers: ML1, TL, and ML2. Thefirst magnetic layer ML1 is disposed over and abutting wordline WL. Thefirst magnetic layer ML1 is often referred to as a hard magnetic layeror a fixed layer because its magnetic orientation is fixed. A tunnellayer or tunnel barrier layer TL comprising a thin dielectric layer isformed over the fixed layer ML1. A second magnetic layer ML2 is formedover the tunnel barrier layer TL. The second magnetic layer ML2 is oftenreferred to as a soft magnetic layer or a free layer because itsmagnetic orientation can be switched along one of two directions. Thefirst and second magnetic layers ML1 and ML2 may include one or morematerial layers.

Each MTJ 102 abuts a second conductive line or bitline BL over andabutting the second magnetic layer ML2, as also shown in FIG. 1, whereinbitline BL is positioned in a direction different from the direction ofthe wordline WL, e.g., the bitlines BL may be perpendicular to thewordlines WL. Array 100 of MTJs 102 includes a plurality of wordlines WLrunning parallel to each other in a first direction, a plurality ofbitlines BL running parallel to each other in a second direction,wherein the second direction is different from the first direction, anda plurality of MTJs 102 disposed between each wordline WL and bitlineBL. While bitlines BL are shown on top and the wordlines WL are shown onbottom of the array 100, alternatively, wordlines WL may be disposed onthe top of the array and bitlines BL may be disposed on the bottom ofthe array.

The value of the resistance of a MTJ 102 depends on the way in which themagnetic moment of the soft magnetic layer ML2 is oriented in relationto the magnetic moment of the hard magnetic layer ML1. The resistance ofMTJ cell 102 depends on the moment's relative alignment. For example, ifthe first and second magnetic layers ML1 and ML2 are oriented in thesame direction, as shown in FIG. 2B, the cell resistance Rc is low. Ifthe first and second magnetic layers ML1 and ML2 are oriented inopposite directions, shown in FIG. 2C, the cell resistance Rc is high.These two states of the MTJ cell are used to store digital information(a logic “1” or “0”, high or low resistance, or vice versa).

The hard magnetic layer ML1 is usually oriented once duringmanufacturing. The information of the cell 102 is stored in the softmagnetic layer ML2. As shown in FIG. 2A, the currents I_(WL) and I_(BL)through the wordline WL and bitline BL, respectively, provide themagnetic field that is necessary to store information in the softmagnetic layer ML2. The superimposed magnetic fields of the bitline BLand wordline WL currents have the ability to switch the magnetic momentof the soft magnetic layer ML2 and change the memory state of MTJ cell102.

An advantageous feature of MRAM devices compared to traditionalsemiconductor memory devices such as dynamic random access memory (DRAM)devices is that MRAM devices are non-volatile. For example, a personalcomputer (PC) utilizing MRAM devices would not have a long “boot-up”time as with conventional PCs that utilize DRAM devices. Also, an MRAMdevice does not need to be powered up and has the capability of“remembering” the stored data (also referred to as a non-volatilememory). MRAM devices have the capability to provide the density of DRAMdevices and the speed of static random access memory (SRAM) devices, inaddition to non-volatility. Therefore, MRAM devices have the potentialto replace flash memory, DRAM, and SRAM devices in electronicapplications where memory devices are needed in the future.

With the down-scaling of integrated circuits, the formation of MRAMdevices experiences problems. After the formation of MTJs 102, therespective wafer is exposed to the external environment. The materialsof MTJs 102 are thus prone to oxidation, and hence the sidewall portionsof MTJs 102 are oxidized. This adversely affects the performance of MTJs102. Particularly, the R-H loop of MTJs 102 will become sloped. Theproblem is further worsened when the dimensions of MTJs 102 are reducedwith the down-scaling of integrated circuits, since the oxidizedportions become greater portions of MTJs 102.

A further problem is the effect of inherent stresses in MTJs 102 totheir performance. After the formation of MTJs 102, process-relatedinherent stresses remain in MTJs 102. It is known that the inherentstresses affect the performance of MTJs 102. The stresses may not be ina desirable direction, or have a desirable magnitude. Theoretically,after the formation of MTJs 102, a post annealing could reduce theinherent stresses. However, this cannot guarantee the reduction of theinherent stresses. New MTJ structures and methods for forming the sameare thus needed to solve the above-discussed problems.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method offorming an integrated circuit includes forming magnetic tunnel junction(MTJ) layers; etching the MTJ layers to form a MTJ cell; and forming adielectric capping layer on sidewalls of the MTJ cell, wherein the stepof forming the dielectric capping layer is in-situ performed with thestep of etching the MTJ layers.

In accordance with another aspect of the present invention, a method offorming an integrated circuit includes providing a semiconductorsubstrate; providing a production tool comprising a first etchingchamber, a second etching chamber, and a deposition chamber; forming abottom electrode layer over the semiconductor substrate; formingmagnetic tunnel junction (MTJ) layers over the bottom electrode layer;forming a hard mask layer over the MTJ layers; in the first etchingchamber, etching the hard mask layer to form a patterned hard masklayer; in the second etching chamber, etching the MTJ layers to form aplurality of MTJ cells, wherein the step of etching the MTJ layers isin-situ performed with the step of etching the hard mask layer; in thedeposition chamber, forming a dielectric capping layer over theplurality of MTJ cells, wherein the dielectric capping layer coverssidewalls of the plurality of MTJ cells, and wherein the step of etchingthe MTJ layers is in-situ performed with the step of forming thedielectric capping layer; and patterning the bottom electrode layer toform a plurality of bottom electrodes over the semiconductor substrate,wherein the plurality of bottom electrodes forms an array.

In accordance with yet another aspect of the present invention, a methodof forming an integrated circuit includes forming MTJ layers; etchingthe MTJ layers to form a plurality of MTJ cells, wherein the pluralityof MTJ cells have inherent stresses; and forming a dielectric cappinglayer on sidewalls of the plurality of MTJ cells. The dielectric cappinglayer has a non-neutral stress compensating for the inherent stresses inthe plurality of MTJ cells.

In accordance with yet another aspect of the present invention, anintegrated circuit includes a MTJ cell; and a dielectric capping layerabutting sidewalls of the MTJ cell. The dielectric capping layer has anon-neutral stress, while an internal stress of the MTJ cell issubstantially neutral.

In accordance with yet another aspect of the present invention, anintegrated circuit include a MTJ cell; a top electrode overlying the MTJcell; and a dielectric capping layer abutting sidewalls of the MTJ cell,wherein the sidewalls of the MTJ cells are substantially un-oxidized.

By forming a dielectric capping layer on MTJ cells before a vacuumbreak, the adverse oxidation of the MTJ cells is substantiallyeliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a perspective view of a conventional magnetoresistiverandom access memory (MRAM) array, with wordlines and bitlines disposedbelow and above each memory cell;

FIGS. 2A through 2C illustrate a single MRAM cell and the currents usedto program the cell;

FIG. 3 illustrates a production tool for forming embodiments of thepresent invention;

FIGS. 4 through 20 illustrate top views and cross-sectional views ofintermediate stages in the manufacturing of embodiments of the presentinvention; and

FIGS. 21A through 21C illustrate top views of exemplary MTJ cells 40 andcorresponding widths W.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A novel magnetic tunnel junction (MTJ) and methods of forming the sameare provided. The intermediate stages of manufacturing embodiments ofthe present invention are illustrated. Throughout the various views andillustrative embodiments of the present invention, like referencenumbers are used to designate like elements.

FIG. 3 illustrates production tool 200 for forming the embodiments ofthe present invention. Production tool 200 includes transfer chamber 202for transferring wafers, etching chambers 204 and 206 for etchingcomponents on the wafers, and deposition chamber 208. Production tool200 further includes loadlock 210 for loading the wafers into, andunloading wafers from, transfer chamber 202. The wafers may betransferred between chambers 204, 206, and 208 through transfer chamber202. During the transferring, chambers 202, 204, 206, and 208 arepreferably kept vacuumed.

Referring to FIG. 4. Wafer 10, which includes substrate 20, is provided.Substrate 20 is preferably a semiconductor substrate, on which activedevices such as transistors (not shown) are formed. The control circuitsand programming circuits (not shown) of magnetoresistive random accessmemory (MRAM) are also formed on substrate 20.

A dielectric layer 22 is formed, through which vias or contacts (notshown) are formed. Dielectric layer 22 may be formed of a low-kdielectric material having a k value less than about 3.9. The k value ofdielectric layer 22 may even be lower than about 2.8. Bottom electrodelayer 24 is then formed over dielectric layer 20. Bottom electrode layer24 may be formed of conductive materials including tantalum, tantalumnitride, or the like. In an exemplary embodiment, bottom electrode layer24 has a thickness in a range about 150 Å to about 350 Å. Although notillustrated, bottom electrode layer 24 includes a plurality ofextensions extending into dielectric layer 22 and electrically connectedto underlying features. In the subsequent patterning of bottom electrodelayer 24, each of the extensions is included into one of the resultingbottom electrodes 25, as is shown in FIG. 14.

FIG. 5 illustrates the formation of MTJ layers 26, the formation of topelectrode layer 28, and the application and patterning of photo resist32. MTJ layers 26 may include various layers formed of differentcombinations of materials. In an exemplary embodiment, MTJ layers 26include pinning layer 26 ₁, synthetic anti-ferro-magnetic layers 26 ₂,26 ₃, and 26 ₄, tunnel barrier layer 26 ₅, and free layer 26 ₆. In anexemplary embodiment, pinning layer 26 ₁ is formed of PtMn with athickness ranging from about 100 Å to about 250 Å (e.g. 150 Å), layer 26₂ is formed of CoFe with a thickness ranging from about 20 Å to about 60Å (e.g. 30 Å), layer 26 ₃ is formed of Ru with a thickness ranging fromabout a few angstroms to about tens of angstroms (e.g. 8 Å), layer 26 ₄is formed of CoFeB with a thickness of about a few tens angstroms (e.g.30 Å), tunnel barrier layer 26 ₅ is formed of MgO with a thicknessranging from about a few angstroms to about a few tens of angstroms(e.g. 10 Å), and free layer 26 ₆ is formed of CoFeB with a thicknessranging from about a few angstroms to about a few tens of angstroms(e.g. 20 Å). Layer 26 ₄ has a magnetic moment in one direction, whichdirection is fixed by pinning layer 26 ₁. The magnetic moment of freelayer 26 ₆ may be programmed, causing the resistivity of the resultingMTJ cell to be changed between a high resistance and a low resistance.

Top electrode layer 28 is formed over MTJ layers 26, and may be formedof tantalum, tantalum nitride, or other conductive materials. Photoresist 32 is then formed and patterned. The remaining portions of photoresist 32 also form an array. Optionally, bottom anti-reflective coating(BARC) 30, such as organic ARC or inorganic ARC, is formed underlyingphoto resist 32. BARC 30 may be formed of silicon oxynitride, siliconrich oxide, nitrogen-free ARC materials including SiOC, or thecombination thereof.

The wafer 10 as shown in FIG. 5 is then loaded into etching chamber 204(refer to FIG. 3). BARC 30 and top electrode layer 28 are then etched,and hence forming top electrodes 36, as shown in FIG. 6. Top electrodes36 also form an array, and are used as a hard mask for the patterning ofthe underlying layers. In an exemplary embodiment wherein top electrodelayer 28 is formed of tantalum, the etchant may includehalogen-containing compound such as CF₄. Next, an ashing is performed toremove photo resist 32 and BARC 30, for example, also in etching chamber204. The resulting wafer 10 is shown in FIG. 7. In an embodiment, theashing process is performed using oxygen-containing gases. In otherembodiments, the ashing process is performed using oxygen-free processgases such as a combined gas of nitrogen and hydrogen. Advantageously,with oxygen-free process gases, the oxidation of free layer 26 ₆ issubstantially eliminated.

Next, wafer 10 is transferred from etching chamber 204 to etchingchamber 206 through transfer chamber 202 (refer to FIG. 3). Sincechambers 202, 204, and 206 are all vacuumed, during the entiretransferring process, wafer 10 is kept in a vacuum environment, and isnot exposed to external detrimental substances such as oxygen, moisture,and the like. MTJ layers 26 are then patterned using patterned ARC layer(not shown) or top electrodes 36 as hard masks. Throughout thedescription, the steps of etching top electrode layer 28 and etching MTJlayers 26 are referred to as “in-situ” performed to indicate that novacuum-break occurs between these two steps. In an exemplary embodiment,the etchant includes alkanols, such as methanol (CH₃OH), ethanol, andbutanol), or a carbon oxide(s) combined with an ammonia-containingcompound(s) (e.g. CO+NH₃). Correspondingly, as shown in FIG. 8, theremaining portions of MTJ layers 26 forms MTJ cells 40, which includeslayers 40 ₁ through 40 ₆, wherein each of the subscripts 1 through 8 areused to indicate features resulting from respective MTJ layers 26.

In an exemplary embodiment, MTJ cells 40 have width W of between about30 nm and about 200 nm, and a combined height H of MTJ cells 40 and therespective overlying top electrodes 36 is about 100 nm. The width W of aMTJ cell 40 is defined as a maximum dimension of the MTJ cell. Exemplarywidths W are shown in FIGS. 21A through 21C, which are top views ofexemplary MTJ cells 40. One skilled in the art will realize, however,that the dimensions recited throughout the description are merelyexamples, and may be reduced with the down-scaling of the integratedcircuits.

Wafer 10 is then transferred to deposition chamber 208 through chamber202 (refer to FIG. 3). Again, since chambers 202, 206, and 208 are allvacuumed, during the transferring process, wafer 10 is free from theadverse effects of the external detrimental substances such as oxygen,moisture, and the like. In deposition chamber 208, dielectric cappinglayer 46 is formed, and the resulting structure is shown in FIG. 9.Throughout the description, the steps of etching MTJ layers 26 andforming dielectric capping layer 46 are referred to as “in-situ”performed to indicate that no vacuum-break occurs between these twosteps. The term “in-situ” may also indicate that between the steps ofetching MTJ layers 26 and forming dielectric capping layer 46, wafer 10is only exposed to environments containing significant lower levels ofoxygen, moisture, than in the open air, even though wafer 10 may beexposed to pressures up to a standard atmosphere. In this case, theweight percentages of oxygen and moisture levels in the environments arepreferably less than about 10 percent the respective weight percentagesof the oxygen and moisture in open air. In the case no vacuum breakoccurs between the steps of etching MTJ layers 26 and forming dielectriccapping layer 46, the partial pressure of oxygen, moisture, and the likeis less than about 15 Torr, and preferable less than about 10⁻³ Torr, oreven less than about 10⁻⁵ Ton. Dielectric capping layer 46 may includecommonly known dielectric materials such as silicon nitride, siliconcarbide, carbon-doped silicon nitride, carbon-doped silicon oxide,silicon oxynitride, and combinations thereof. Dielectric capping layer46 may also be a composite layer including two or more layers, forexample, layers 46 ₁ and 46 ₂, which are formed of different materialssuch as a silicon nitride/silicon carbine stack. In this case, layer 46₂ may be formed in an environment containing significant level ofoxygen, moisture, and the like.

FIG. 10 illustrates the patterning of dielectric capping layer 46 andbottom electrode layer 24. The remaining portions of dielectric cappinglayer 46 cover the top and sidewalls of each of the MTJ cells 40. Bottomelectrode layer 24 is patterned to form bottom electrodes 25. Pleasenote that although not illustrated, each of the bottom electrodes 25 mayinclude a portion extending into dielectric layer 22, and electricallyconnected to underlying features (refer to FIG. 20 for details).

MTJ cells 40 may have processed-induced inherent stresses. Typically,MTJ cells 40 each have an easy axis (for example, a long axis if the MTJcells have the shape of ellipses if viewed from top). If the magneticmoments are well aligned to the easy axis, the moment-switching of theMTJ cells may be performed coherently, and hence the switchinguniformity may be increased. However, the inherent stresses of the MTJcells 40 may cause the easy axis to deviate from the expected direction,for example, the long axis. As a result, it is difficult to predict theeasy axis and to align the easy axis to the desirable direction.Dielectric capping layer 46 thus preferably has an inherent stress,preferably no greater than about 3 GPa, and hence applies a stress tocompensate for the inherent stress in MTJ cells 40. In an embodiment ofthe present invention, due to the compensation, the stress in MTJ cells40 is at least reduced, and possibly substantially eliminated, forexample, to about 1 MPa or less. Throughout the description, a featurehaving substantially no compressive and no tensile stress is referred toas being neutral. In an exemplary embodiment, if MTJ cells 40 haveinternal compressive stresses, dielectric capping layer 46 preferablyhas a tensile stress. Conversely, if MTJ cells 40 have internal tensilestresses, dielectric capping layer 46 preferably has a compressivestress. In other embodiments, the stress in dielectric capping layer 46actually increases the stress in MTJ cell 40. The net result is to alignthe easy axis to the desirable direction and to improve the performanceof MTJ cells 40.

Preferably, the thickness T of dielectric capping layer 46 is greatenough to apply suitable stresses and to provide insulations. On theother hand, the thickness T should not be too great to cause processdifficulty. As one skilled in the art will perceive, dielectric cappinglayer 46 needs to be thicker if the intrinsic stress of dielectriccapping layer 46 is smaller, or the inherent stresses of MTJ cells 40are greater. Preferably, thickness T is great enough such that aresulting spacer width W′ (refer to FIGS. 11 and 12) after a subsequentremoval process to dielectric capping layer 46 is no greater about twotimes of width W of MTJ cells 40. In an exemplary embodiment, thicknessT of dielectric capping layer 46 is between about 20 Å and about 1 kÅ,depending on the intrinsic stress of dielectric capping layer 46 and theinherent stresses of MTJ cells 40. Experiments may be performed todetermine the optimum stress and optimum thickness of dielectric cappinglayer 46.

One of ordinary skilled in the art will recognize that the stressgenerated in dielectric capping layer 46 is related to the processconditions, and a wide range of compressive and tensile stresses can begenerated by the same dielectric material. For instance, silicon nitridedeposited by low-pressure chemical vapor deposition (LPCVD) may have atensile stress. Likewise, silicon nitride deposited by plasma-enhancedchemical vapor deposition (PECVD) may have a compressive stress. Also,even formed using a same method and a same material, the processconditions, such as temperatures may also affect the stress in theresulting dielectric capping layer 46. Preferably, the depositiontemperature for forming dielectric capping layer 46 is lower than about450° C., such as between about 200° C. and 350° C. The depositiontemperature may even be between about 20° C. and about 120° C. The lowdeposition temperature advantageously reduces the inter-diffusionbetween the layers of MTJ cells 40, and hence their performance may beimproved. The other applicable formation methods for forming dielectriccapping layer 46 include spin-on coating, low temperature CVD, laserablation deposition, and the like. In the case dielectric capping layer46 includes more than one layer, for example, layers 46 ₁ and 46 ₂formed of different materials, layers 46 ₁ and 46 ₂ may have differentstresses.

Protected by dielectric capping layer 46, MTJ cells 40 are insulatedfrom external detrimental substances such as oxygen and moisture, andhence can be taken out of the vacuum environment without causing theoxidation of the sidewalls of MTJ cells 40. In FIG. 11, dielectriccapping layer 46 is patterned to form openings 48, exposing a portion ofeach of top electrodes 36. In subsequent steps, contacts may be formedin openings 48.

FIGS. 12 and 13 illustrate another embodiment for exposing topelectrodes 36. In FIG. 12, an additional dielectric layer 50, which maybe an oxide layer comprising carbon-containing silicon oxide,fluorine-containing silicon oxide, silicon carbine, or a low-kdielectric material having a dielectric constant less than about 3.2(such as polyimide and the like) used for forming inter-metaldielectrics, is deposited over dielectric capping layer 46, and the gapsbetween MTJ cells 40 are fully filled. Dielectric layer 50 is preferablyformed of a different material than that of dielectric capping layer 46,although they can be the same. The same dielectric layer 50 may also begap-filled into the structure shown in FIG. 11. In FIG. 13, a removalprocess, such as etch back or a chemical mechanical polish (CMP), isperformed, until the top surfaces of top electrodes 36 are exposed.Preferably, the top edges 52 of the remaining portions of dielectriccapping layer 46 are higher than the top surfaces of MTJ cells 40, sothat none of the MTJ cells 40 are exposed to the external environment.The difference D is preferably between about ⅕ and about ¾ of thecombined height H of top electrodes 36 and MTJ cells 40 to ensure thetop surfaces of top electrodes 36 are exposed, while no sidewalls of MTJcells 40 are exposed.

FIGS. 14 and 15 illustrate another embodiment for exposing topelectrodes 36. Referring to FIG. 14, optional etch stop layer 51 and anadditional dielectric layer 53 are blanket formed. Dielectric layer 53may be formed of essentially the same material as dielectric layer 51,which may include a carbon-containing silicon oxide, a silicon carbine,a nitrogen-containing oxide, a silicon nitride, a silicon oxynitride, ora dielectric material having a dielectric constant less than about 4.2.Dielectric layer 53 may also be formed of silicon oxide, or polyimide. ACMP is performed to the dielectric layer 53, until a desirable thicknessis achieved. Next, as shown in FIG. 15, via openings 55 are formed byetching through dielectric layer 53 etch stop layer 51 to expose the topsurfaces of top electrodes 36. Contact plugs (not shown) may then beformed.

FIGS. 16 and 17 illustrate yet another embodiment for exposing topelectrodes 36. The initial structure of this embodiment may beessentially the same as shown in FIG. 12, in which the additionaldielectric layer 50 (used for forming inter-metal dielectrics) isdeposited over dielectric capping layer 46, and the gaps between MTJcells 40 are fully filled. Next, in FIG. 16, a CMP is performed until apredetermined thickness. Dielectric layer 50 is then etched to formvia/contact openings 80, wherein dielectric capping layer 46 acts as anetch stop layer. Next, as shown in FIG. 17, dielectric capping layer 46is etched through via/contact openings 80, exposing the top surfaces oftop electrodes 36. Vias/contacts 82 are then formed in via/contactopenings 80.

FIGS. 18 and 19 illustrate yet another embodiment for exposing topelectrodes 36. The initial structure of this embodiment may beessentially the same as shown in FIG. 12, in which the additionaldielectric layer 50 (used for forming inter-metal dielectrics) isdeposited over dielectric capping layer 46, and the gaps between MTJcells 40 are fully filled. A CMP is performed until the top surface ofdielectric capping layers 46 are exposed. In the CMP, dielectric cappinglayers 46 act as CMP stop layers. The resulting structure is shown inFIG. 18. Next, as shown in FIG. 19, dielectric capping layer 46 isetched, forming openings 86. Via/contacts (not shown) will be formed inopenings 86 in subsequent process steps.

FIG. 20 illustrates a cross-sectional view of an exemplary embodimentillustrating the position of MTJ cells 40 in wafer 10. In theillustrated exemplary embodiment, MTJ cells 40 are formed in adielectric layer over the third metallization layer (M3). One skilled inthe art will realize that MTJ cell 40 may be formed in a dielectriclayer over other metallization layers. Through vias and metal pads inthe bottom metallization layer (Ml) through metallization layer M3,bottom electrode 25 connects the MTJ cells 40 to a control transistor.Top contact 58 connects top electrode 36 and MTJ cell 40 to writebitline WBL. Write wordline WWL is formed underlying MTJ 40, and isseparated from bottom electrode 25 by insulation layer 60.

The embodiments of the present invention have several advantageousfeatures. First, by forming dielectric capping layers to protect thesidewalls of MTJ cells before the MTJ cells are exposed to externalenvironment, the adverse oxidation of the sidewalls, and hence theadverse performance degradation of MTJ cells, is substantiallyeliminated. This is particularly helpful for MTJ cells formed usingadvanced technologies, for example, 65 nm and below, in which theoxidized portions may be a significant portion of the MTJ cells.Experiments have revealed that the R-H loops of the MTJ cells aresubstantially perfect squares, indicating excellent switching ability.Second, the stresses of the dielectric capping layers are adjusted, andhence better MTJ performance, such as higher MR ratio, smaller writingcurrent, and better reliability assurance uniformity may be obtained inboth cross-type and spin torque transfer (STT) type MRAM applications.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A device comprising: a magnetic tunnel junction (MTJ) cellcomprising: a pinning layer; a tunnel barrier layer; and a free layer;and a dielectric capping layer on sidewalls of the MTJ cell comprisingsidewalls of the pinning layer, the tunnel barrier layer, and the freelayer, wherein the dielectric capping layer has a non-neutral stress. 2.The device of claim 1, wherein an internal stress of the MTJ cell is nogreater than about 1 MPa.
 3. The device of claim 1, wherein thenon-neutral stress in the dielectric capping layer has a magnitude lessthan about 3 GPa.
 4. The device of claim 1 further comprising a topelectrode over the MTJ cell, wherein a top edge of the dielectriccapping layer is lower than a top surface of the top electrode, andhigher than a bottom surface of the top electrode.
 5. The device ofclaim 1, wherein the dielectric capping layer comprises materialsselected the group consisting essentially of nitrogen-containingcompounds and carbon-containing compounds.
 6. The device of claim 1,wherein the dielectric capping layer is a composite layer comprisingmore than one layer formed of different materials.
 7. The device ofclaim 1, wherein the dielectric capping layer has a thickness no greaterthan about two times a width of the MTJ cell.
 8. The device of claim 1,wherein edges of the MTJ cell are substantially free from oxidation. 9.The device of claim 1, wherein the dielectric capping layer furthercomprises a portion directly over and vertically aligned to the tunnelbarrier layer.
 10. The device of claim 1 further comprising a bottomelectrode underlying and electrically coupled to the MTJ cell, whereinthe dielectric capping layer further comprises a portion directly overand contacting the bottom electrode.
 11. A device comprising: a magnetictunnel junction (MTJ) cell comprising: a pinning layer; a tunnel barrierlayer; and a free layer; a top electrode overlying the MTJ cell; and adielectric capping layer on sidewalls of the MTJ cell comprisingsidewalls of the pinning layer, the tunnel barrier layer, and the freelayer, wherein the sidewalls of the MTJ cell are substantiallyun-oxidized.
 12. The device of claim 11, wherein the dielectric cappinglayer is a composite layer comprising more than one layer formed ofdifferent materials.
 13. The device of claim 11, wherein the dielectriccapping layer has a non-neutral stress.
 14. The device of claim 13,wherein the non-neutral stress in the dielectric capping layer has amagnitude less than about 3 GPa.
 15. A device comprising: a bottomelectrode; a magnetic tunnel junction (MTJ) cell over the bottomelectrode; a top electrode over the MTJ cell; and a dielectric cappinglayer comprising: a top portion directly over the top electrode; asidewall portion on a sidewall of the MTJ cell; and a bottom portiondirectly over the bottom electrode and vertically misaligned with thetop portion and the sidewall portion, wherein the top portion, thesidewall portion, and the bottom portion form a continuous region. 16.The device of claim 15 further comprising; a pinning layer; a tunnelbarrier layer; and a free layer, wherein the pinning layer, the tunnelbarrier layer, the free layer, and the top electrode are substantiallyco-terminus with respective edges vertically aligned to each other. 17.The device of claim 15, wherein a bottom surface of the top portioncontacts a top surface of the top electrode, and a bottom surface of thebottom portion contacts a top surface of the bottom electrode.
 18. Thedevice of claim 15, wherein the dielectric capping layer has a thicknessno greater than about two times a width of the MTJ cell.
 19. The deviceof claim 15, wherein edges of the MTJ cell are substantially free fromoxidation.
 20. The device of claim 15, wherein an internal stress of theMTJ cell is no greater than about 1 MPa.